System.Net.Http.CurlHandler.SslProvider.SetSslVersion C# (CSharp) Method

            private static void SetSslVersion(EasyRequest easy, IntPtr sslCtx = default(IntPtr))
            {
                // Get the requested protocols.
                SslProtocols protocols = easy._handler.SslProtocols;
                if (protocols == SslProtocols.None)
                {
                    // Let libcurl use its defaults if None is set.
                    return;
                }

                // We explicitly disallow choosing SSL2/3. Make sure they were filtered out.
                Debug.Assert((protocols & ~SecurityProtocol.AllowedSecurityProtocols) == 0, 
                    "Disallowed protocols should have been filtered out.");

                // libcurl supports options for either enabling all of the TLS1.* protocols or enabling 
                // just one of them; it doesn't currently support enabling two of the three, e.g. you can't 
                // pick TLS1.1 and TLS1.2 but not TLS1.0, but you can select just TLS1.2.
                Interop.Http.CurlSslVersion curlSslVersion;
                switch (protocols)
                {
                    case SslProtocols.Tls:
                        curlSslVersion = Interop.Http.CurlSslVersion.CURL_SSLVERSION_TLSv1_0;
                        break;
                    case SslProtocols.Tls11:
                        curlSslVersion = Interop.Http.CurlSslVersion.CURL_SSLVERSION_TLSv1_1;
                        break;
                    case SslProtocols.Tls12:
                        curlSslVersion = Interop.Http.CurlSslVersion.CURL_SSLVERSION_TLSv1_2;
                        break;

                    case SslProtocols.Tls | SslProtocols.Tls11 | SslProtocols.Tls12:
                        curlSslVersion = Interop.Http.CurlSslVersion.CURL_SSLVERSION_TLSv1;
                        break;

                    default:
                        throw new NotSupportedException(SR.net_securityprotocolnotsupported);
                }

                try
                {
                    easy.SetCurlOption(Interop.Http.CURLoption.CURLOPT_SSLVERSION, (long)curlSslVersion);
                }
                catch (CurlException e) when (e.HResult == (int)CURLcode.CURLE_UNKNOWN_OPTION)
                {
                    throw new NotSupportedException(SR.net_securityprotocolnotsupported, e);
                }
            }